Embryology

Animal Sciences
COPYRIGHT 2002 The Gale Group Inc.

Embryology

Embryology is the biological field of study that examines the early development of organisms. In general, a developing organism is considered an embryo until the point at which all the essential tissues and organ systems have developed. In humans, the embryonic stage covers approximately the first two months of pregnancy.

Certain key events occur during the embryonic development of all multicellular animals. These include fertilization, the union of the sperm and egg to form the fertilized egg, or zygote ; cleavage, when the fertilized egg divides in organized cycles to produce multicellularity; gastrulation, in which the three primary germ layers, the ectoderm, mesoderm, and endoderm, are differentiated; and finally organogenesis, during which the organs develop.

Two general approaches are often taken in the study of embryology: These are descriptive embryology and experimental embryology. Descriptive embryology dates from antiquity, and attempts to describe the normal sequence of developmental events that occur during embryonic development in a given organism. This information can be used to explain how adult anatomy is achieved. Understanding normal development also allows scientists to understand the origin of common birth defects.

Experimental embryology attempts to shed light on the basic processes involved in development, particularly at the cellular level. Experimental embryologists want to discover how development is controlled and how ever more complex structures and organs are produced. They tend to focus on one of a few model organisms about which considerable developmental and genetic information is already known. These model organisms include the mouse, the chicken, the fruit flyDrosophila melanogaster, the African clawed frog Xenopus laevis, and the nematode Caenorhabditis elegans. One particularly effective approach in experimental embryology has been that of developmental genetics, which studies the effect of mutant genes on developmental processes. By comparing the developmental results of mutant versus normal genes, the role of individual genes in development can be assessed. This experimental strategy usually involves screening large numbers of animals for developmental abnormalities.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

embryology

embryology The study of the development of animals from the fertilized egg to the new adult organism. It is sometimes limited to the period between fertilization of the egg and hatching or birth (see embryo).

Cite this article Pick a style below, and copy the text for your bibliography.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Embryology

Embryology

Embryology is the study of the development of organisms. This is as true of plants as it is of animals. Seed formation proceeds following fertilization in higher plants. The seed consists of the embryo, the seed coat, and another part sometimes called the endosperm. While plants are extraordinarily important for survival of animal life, animal embryology is described here.

The dictionary definition limits the meaning of “embryo” to developing animals that are unhatched or not yet born. The human embryo phase is the first eight weeks after conception. Many embryologists

have difficulty with this terminology because it is purely arbitrary. It would be difficult indeed, if not impossible, to distinguish a human embryo nearing the end of the eighth week from one in the ninth week after conception. Correspondingly, there are no morphological events that distinguish a prehatched tadpole from a posthatched one (hatching never occurs synchronously in an egg mass—there are always larvae that hatch early and those that are dilatory).

Embryologists study an embryo’s development from a zygote to a multicellular organism. In the particular case of humans, development does not even stop at birth: teeth continue to develop and sex glands with sexual differentiation mature long after birth. Many embryologists refer to their discipline as developmental biology to escape from the need to confine their studies to earlier stages. Embryology in the modern sense is the study of an animal’s life history; human embryology considers developmental aspects of life as a whole and not just the first eight weeks.

The study of embryology, the science that deals with the formation and development of the embryo and fetus, can be traced to ancient Greek philosophers. Originally, embryology was part of the field known as “generation,” a term that also encompassed studies of reproduction, development and differentiation, regeneration of parts, and genetics. Generation describes the means by which new animals or plants come into existence. The ancients believed that new organisms could arise through sexual reproduction, asexual reproduction, or spontaneous generation. As early as the sixth century BC, Greek physicians and philosophers suggested studying the developing chicken egg to investigate embryology.

Aristotle (384–322 BC) described the two historically important models of development known as pre-formation and epigenesis. According to preformation theory, an embryo or miniature individual preexists in either the mother’s egg or the father’s semen and begins to grow when properly stimulated. Some preformationists believed that all the embryos that would ever develop had been formed by God at creation. Aristotle actually favored the theory of epigenesis, which assumed that the embryo begins as an undifferentiated mass and that new parts are added during development. Aristotle thought that the female parent contributed only unorganized matter to the embryo. He argued that semen from the male parent provided the “form,” or soul, which guided development and that the first part of the new organism to be formed was the heart.

Aristotle’s theory of epigenetic development dominated embryology until physiologist William Harvey (1578-1657) raised doubts about many aspects of classical theories. In his studies of embryology, as in his research on the circulation of the blood, Harvey was inspired by the work of his teacher, Girolamo Fabrici (ca.1533-1619). Some historians think that Fabrici should be considered the founder of modern embryology because of the importance of his embryological texts: On the Formed Fetus and On the Development of the Egg and the Chick. Harvey’s On the Generation of Animals was not published until 1651, but it was the result of many years of research. Although Harvey began these investigations to provide experimental proof for Aristotle’s theory of epigenesis, his observations proved that many aspects of Aristotle’s theory of generation were wrong.

Aristotle believed that the embryo essentially formed by coagulation in the uterus immediately after mating when the form-building principle of themale acted on the material substance provided by the female. Using deer that had mated, Harvey dissected the uterus and searched for the embryo. He was unable to find any signs of a developing embryo in the uterus until about six or seven weeks after mating had taken place. In addition to his experiments on deer, Harvey carried out systematic studies of the developing chick egg. His observations convinced him that generation proceeded by epigenesis, that is, the gradual addition of parts. Nevertheless, many of Harvey’s followers rejected epigenesis and turned to theories of preformation.

Naturalists who favored preformation were inspired by the new mechanical philosophy and by the microscope, a device that allowed them to see the embryo at earlier stages of development. Some naturalists produced very unreliable observations of early embryos, but Marcello Malpighi (1628-1694) and Jan Swammerdam (1637-1680), two pioneers of microscopy, provided observations that seemed to support preformation. Based on Swammerdam’s studies of insects and amphibians, naturalists suggested that embryos preexisted within each other like a nest of boxes. However, given such a theory, only one parent can serve as the source of the sequence of preformed individuals. At the time, the eggs of many species were well known, but when the microscope revealed the existence of “little animals” in male semen, some naturalists argued that the preformed individuals must be present in the sperm.

Respected scientists of the time, including Albrecht von Haller (1708-1777), Charles Bonnet (1720-1793), Lazzaro Spallanzani (1729-1799), and Rene´ Antoine Ferchault de Reaumur (1683-1757), supported preformation. Bonnet’s studies of parthenogenesis in aphids were regarded as strong support of ovist preformationism. Thus, some naturalists argued that the whole human race had preexisted in the ovaries of Eve, while others reported seeing homunculi (tiny people) inside spermatozoa. Other eighteenth-century naturalists rejected both ovist and spermist preformationist views. One of the most influential was Casper Friedrich Wolff (1733-1794), who published a landmark article in the history of embryology, “Theory of Generation,” in 1759. Wolff argued that the organs of the body did not exist at the beginning of gestation, but formed from some originally undifferentiated material through a series of steps. Naturalists who became involved in the movement known as nature philosophy found Wolff’s ideas very attractive. During the nineteenth century, cell theory, the discovery of the mammalian ovum by Karl Ernst von Baer (1792-1876), and the establishment of experimental embryology by Wilhelm Roux (1850-1924) and Hans Driesch (1867-1941) transformed philosophical arguments about the nature of embryological development.

About a century ago, a number of developing organisms were observed carefully. By this time, there was a cell theory and good microscopes were available. Next came a causal analysis. For instance, it was known that the dorsal ectoderm of all vertebrate embryos rolls up into a tube to form the central nervous system. What factors control the very regular appearance of the nervous system and subsequent differentiation into the various parts of the brain and the spinal cord? It was hypothesized that the underlying chordamesoderm cells of the gastrula signaled the ectoderm to become neural. The signal was referred to as induction. Other embryonic organs also seemed to appear as a result of induction.

Chemical embryology sought to characterize the nature of inducing signals. Modern molecular embryology studies specific tissue and cell differentiation at the genetic level.

There are practical considerations that drive some embryologists. The causes of developmental abnormalities (congenital malformations) in humans becomes more understandable with a consideration of embryology. The human embryo is extraordinarily vulnerable to drugs, viruses, and radiation during the first several months of development when many critical organ systems are developing.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.

Embryology

The Gale Encyclopedia of Science
COPYRIGHT 2008 The Gale Group, Inc.

Embryology

Embryology is the study of the development of organisms. This is as true of plants as it is of animals.

Seed formation proceeds following fertilization in higher plants. The seed consists of the embryo, the seed coat, and another part sometimes called the endosperm. While plants are extraordinarily important for survival of animal life, animal embryology is described here.

The dictionary definition limits the meaning of the term "embryo" to developing animals that are unhatched or not yet born. Human embryos are defined as developing humans during the first eight weeks after conception. The reason that many embryologists have difficulty with this terminology is that it is purely arbitrary. It would be difficult indeed, if not impossible, to discriminate a human embryo nearing the end of the eighth week from a developing human during the ninth week after conception. Correspondingly, there are no morphological events that distinguish a pre-hatching frog tadpole from a post-hatching tadpole (hatching never occurs synchronously in an egg mass—there are always those that hatch early and those larvae which are dilatory).

Embryologists consider development from a zygote to a multicellular organism . In the particular case of humans, development does not even stop at birth . Note that teeth continue to develop and sex glands with sexual differentiation mature long after birth. For a number of years, many embryologists have referred to their discipline as developmental biology to escape from the need to confine their studies to earlier stages. Embryology in the modern sense is the study of the life history of an animal and human embryology considers developmental aspects of life as a whole and not just the first eight weeks.

History of embryology as a science

The study of embryology, the science that deals with the formation and development of the embryo and fetus, can be traced back to the ancient Greek philosophers. Originally, embryology was part of the field known as "generation," a term that also encompassed studies of reproduction, development and differentiation, regeneration of parts, and genetics . Generation described the means by which new animals or plants came into existence. The ancients believed that new organisms could arise through sexual reproduction , asexual reproduction , or spontaneous generation . As early as the sixth century b.c., Greek physicians and philosophers suggested using the developing chick egg as a way of investigating embryology.

Aristotle (384–322 b.c.) described the two historically important models of development known as preformation and epigenesis. According to preformationist theories, an embryo or miniature individual preexists in either the mother's egg or the father's semen and begins to grow when properly stimulated. Some preformationists believed that all the embryos that would ever develop had been formed by God at the Creation. Aristotle actually favored the theory of epigenesis, which assumes that the embryo begins as an undifferentiated mass and that new parts are added during development. Aristotle thought that the female parent contributed only unorganized matter to the embryo. He argued that semen from the male parent provided the "form," or soul, that guided development and that the first part of the new organism to be formed was the heart .

Aristotle's theory of epigenetic development dominated the science of embryology until the work of physiologist William Harvey (1578–1657) raised doubts about
many aspects of classical theories. In his studies of embryology, as in his research on the circulation of the blood , Harvey was inspired by the work of his teacher, Girolamo Fabrici (ca.1533–1619). Some historians think that Fabrici should be considered the founder of modern embryology because of the importance of his embryological texts: On the Formed Fetus and On the Development of the Egg and the Chick. Harvey's On the Generation of Animals was not published until 1651, but it was the result of many years of research. Although Harvey began these investigations in order to provide experimental proof for Aristotle's theory of epigenesis, his observations proved that many aspects of Aristotle's theory of generation were wrong.

Aristotle believed that the embryo essentially formed by coagulation in the uterus immediately after mating when the form-building principle of the male acted on the material substance provided by the female. Using deer that had mated, Harvey dissected the uterus and searched for the embryo. He was unable to find any signs of a developing embryo in the uterus until about six or seven weeks after mating had taken place. In addition to his experiments on deer, Harvey carried out systematic studies of the developing chick egg. His observations convinced him that generation proceeded by epigenesis, that is, the gradual addition of parts. Nevertheless, many of Harvey's followers rejected epigenesis and turned to theories of preformation.

Naturalists who favored preformationist theories of generation were inspired by the new mechanical philosophy and by the microscope , a device that allowed them to see the embryo at earlier stages of development. Some naturalists produced very unreliable observations of early embryos, but Marcello Malpighi (1628–1694) and Jan Swammerdam (1637–1680), two pioneers of microscopy , provided observations that seemed to support preformation. Based on Swammer dam's studies of insects and amphibians , naturalists suggested that embryos preexisted within each other like a nest of boxes. However, given such a theory, only one parent can serve as the source of the sequence of preformed individuals. At the time, the egg of many species was well known, but when the microscope revealed the existence of "little animals" in male semen, some naturalists argued that the preformed individuals must be present in the sperm.

Respected scientists of the time, including Albrecht von Haller (1708–1777), Charles Bonnet (1720–1793), Lazzaro Spallanzani (1729–1799), and René Antoine Ferchault de Reaumur (1683–1757), supported preformation. Bonnet's studies of parthenogenesis in aphids were regarded as strong support of ovist preformationism. Thus, some naturalists argued that the whole human race had preexisted in the ovaries of Eve, while others reported seeing homunculi (tiny people) inside spermatozoa. Other eighteenth century naturalists rejected both ovist and spermist preformationist views. One of the most influential was Casper Friedrich Wolff (1733–1794), who published a landmark article in the history of embryology, "Theory of Generation," in 1759. Wolff argued that the organs of the body did not exist at the beginning of gestation, but formed from some originally undifferentiated material through a series of steps. Naturalists who became involved in the movement known as nature philosophy found Wolff's ideas very attractive. During the nineteenth century, cell theory, the discovery of the mammalian ovum by Karl Ernst von Baer (1792–1876), and the establishment of experimental embryology by Wilhelm Roux (1850–1924) and Hans Driesch (1867–1941) transformed philosophical arguments about the nature of embryological development.

About a century ago, careful observations were made of a number of developing organisms. By this time, there was a cell theory and good microscopes were available. Next came a causal analysis. For instance, it was known that the dorsal ectoderm of all vertebrate embryos rolls up into a tube to form the central nervous system . What factors control the very regular appearance of the nervous system and subsequent differentiation into the various parts of the brain and the spinal cord? It was hypothesized that the underlying chordamesoderm cells of the gastrula signaled the ectoderm to become neural. The signal was referred to as induction. Other embryonic organs also seemed to appear as a result of induction. Chemical embryology sought to characterize the nature of inducing signals. Now, modern molecular embryology seeks to examine on the level of the gene what controls differentiation of specific tissue and cell typed of a developing organism.

There are practical considerations that drive some embryologists. The causes of developmental abnormalities (congenital malformations) in humans becomes more understandable with a consideration of embryology. The human embryo is extraordinarily vulnerable to drugs, viruses, and radiation during the first several months of development when many critical organ systems are developing.

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

Notes:

Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.

In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.